An Empirical Model for B-Site Cation Ordering in Ba(Mg\u3csub\u3e1/3\u3c/sub\u3eTa\u3csub\u3e2/3\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e

Processing-structure models are needed in both the lab and industry; however, few exist for cation ordering in perovskites. The perovskite Ba(Mg1/3Ta2/3)O3 in its ordered form is one of the best known high-Q dielectric materials but requires extended high-temperature annealing to achieve high degrees of order; so an empirical model which describes the ordering as a function of an easily obtainable processing parameter would be useful. In this work, powders of Ba(Mg1/3Ta2/3)O3 were synthesized using a conventional solid-state mixed-oxide method. The as-calcined compound had a cubic (lacking long-range B-site cation order) structure but contained short-range-ordered nanodomains. Upon annealing at 1500 °C for up to 40 h an increasingly ordered arrangement of Mg2+ and Ta5+ on the B site was generated, with the ordering causing a trigonal distortion. Empirical modeling as well as first-principles calculations via density functional theory showed that this ordering process was accompanied by a volume decrease despite the fact that ordered planes stack less efficiently. An empirical model was developed to describe the ordering parameter as a function of either annealing time or effective B-site contraction. The implication of this modeling method is that it may be possible to predict the degree of cation ordering in complex perovskite systems from ionic-radii data and experimentally-derived pseudocubic lattice constants alone. Conversely, it may also be possible to predict the degree of volume expansion/contraction upon ordering, which has implications for functional properties like ionic conduction

Fabrication of Stoichiometric U\u3csub\u3e3\u3c/sub\u3eSi\u3csub\u3e2\u3c/sub\u3e Fuel Pellets

Uranium silicide, U3Si2, is an accident tolerant fuel type which is gaining momentum as a replacement fuel for uranium dioxide (UO2). Idaho National Laboratories has been fabricating phase pure U3Si2 fuel pellets for use in various irradiation and material characterization experiments. Stoichiometric U3Si2 fuel pellets were fabricated using a powder metallurgy and arcmelting technique. The use of the stoichiometric ratio to alloy uranium and silicon, and sintering in a vacuum environment allowed for the fabrication of high density (\u3e94% theoretical density), phase pure pellets, greater than 94% U3Si2. Silicon volatilization was not observed in the as-sintered microstructure, which has been verified through XRD and SEM, thus eliminating the need to alloy a substoichiometric U/Si ratio. Stoichiometric ratio of U to Si used to form U3Si2 phase. Decrease in secondary phases present confirm absence of silicon volatilization. Analysis via XRD and SEM confirm the phase purity of the U3Si2 fuel pellets

Stable isotope evidence for magmatic fluid input during large-scale Na–Ca alteration in the Cloncurry Fe oxide Cu–Au district, NW Queensland, Australia

Sodic–calcic alteration is common in mineralized hydrothermal systems, yet the relative importance of igneous vs. basinal fluid sources remains controversial. One of the most extensive volumes of sodic–calcic rocks occurs near Cloncurry, NW Queensland, and was formed by overlapping hydrothermal systems that were active synchronously with emplacement of mid-crustal batholithic granitoids (c. 1.55–1.50 Ga). Altered rocks contain albite–oligoclase, actinolite, diopside, titanite and magnetite. Alteration was localized by: (A) composite veins and breccias containing crystallized magma intimately intergrown with hydrothermal precipitates; (B) intrusions that host setting A veins and breccias; and (C) extensive breccia and vein systems linked to regional fault systems. Isotope analyses of actinolites in settings A and B indicate calculated δ18OH2O (+8.2 to +10.6‰) and variably depleted δDH2O (−130 to −54‰) compared with typical magmatic fluids, whereas those from setting C typically indicate calculated δ18OH2O (+8.0 to +12.8‰) and δDH2O (−29 to −99‰). The lowest δDH2O values are interpreted as representing residual fluids after significant (> 90%) open-system magmatic degassing. Overall the stable isotope, field, geochronological and geobarometric data suggest that these sodic–calcic alteration systems were formed by the episodic incursion of magmatic fluids that underwent minor isotopic modification as a result of varying degrees of interaction with country rocks

An Empirical Model for Perovskite Tetragonality

Tetragonal distortions in perovskites are useful for advanced devices, as they can induce functional properties like capacitance, piezoelectricity, pyroelectricity, and ferroelectricity. These distortions are commonly due to either second-order Jahn-Teller effects or antiferrodistortive instabilities. Accurate empirical composition-structure models can significantly reduce the time and cost involved with developing new functional ceramics; however, until now no such model has existed for tetragonal perovskites, the structures of which are further complicated by extrinsic point defects often used to tune electrical properties. In this work, LeBail refinements of X-ray diffraction patterns obtained from [(PbyBa1−y)(1−3x)La(2x)]TiO3 (P4mm, No. 99) were conducted to measure lattice constants and reveal tetragonality trends. These trends were combined with a model for unit-cell volume in order to derive a general empirical model, based solely on published ionic radii data, for tetragonal lattice constants of perovskites in P4mm and P4/mmm

Structural effect of aliovalent Doping in lead perovskites

Composition–structure relationships are needed for various applications, including lattice-matching for heteroepitaxy; however, a general model to predict lattice constants in defective perovskites is not yet available because the exact nature of A-site vacancies in perovskites remains largely unknown. In this study, it has been shown experimentally via Le Bail refinements of x-ray diffraction data that such vacancies in (Pb1−3xLa2x□x)TiO3 and (Pb1−3xLa2x□x)(Zr0.6Ti0.4)O3 have an effective size due to both Coulombic repulsion of coordinating oxygen ions and bond relaxation. For the first time, cell volume can be predicted in this system from stoichiometry and published ionic radii data alone to within 0.2% accuracy within View the MathML source0≤x≤13 compositional range and <0.3% accuracy all the way to View the MathML sourcex=13. The model may be applied to other perovskite systems and eventually provide tailored properties (magnetic, dielectric, and other) based on improved structure predictions

Cellular and molecular defects in a patient with Hermansky-Pudlak syndrome type 5

<div><p>Hermansky-Pudlak syndrome (HPS) is a heterogeneous group of genetic disorders typically manifesting with tyrosinase-positive oculocutaneous albinism, bleeding diathesis, and pulmonary fibrosis, in some subtypes. Most HPS subtypes are associated with defects in Biogenesis of Lysosome-related Organelle Complexes (BLOCs), which are groups of proteins that function together in the formation and/or trafficking of lysosomal-related endosomal compartments. BLOC-2, for example, consists of the proteins HPS3, HPS5, and HPS6. Here we present an HPS patient with defective BLOC-2 due to a novel intronic mutation in <i>HPS5</i> that activates a cryptic acceptor splice site. This mutation leads to the insertion of nine nucleotides in-frame and results in a reduced amount of HPS5 at the transcript and protein level. In studies using skin fibroblasts derived from the proband and two other individuals with HPS-5, we found a perinuclear distribution of acidified organelles in patient cells compared to controls. Our results suggest the role of HPS5 in the endo-lysosomal dynamics of skin fibroblasts.</p></div